<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      let num = 100; //全局
      function fn() {
        let username = "jack";
        console.log(username); //jack
        function fm() {
          let age = 20;
          console.log(age); //20
          console.log(username); //jack
        }
        fm();
      }
      fn();
      // console.log(username); //error
    </script>
  </body>
</html>
